While immune cells play a critical role in early innate immune responses and microbial clearance during infection, excessive immune cell activity and the inflammatory host responses this activation may elicit, can also lead to inadvertent host tissue injury. The host has therefore developed mechanisms to control and coordinate immune cell activity. The programed death 1 (PD-1) signaling pathway is one such mechanism. The development of PD-1 expression on lymphocytes and its stimulation by PD-1 ligand is associated with a suppressed immune phenotype. Although the suppressed state associated with PD-1 expression is the hosts attempt to keep an excessive immune response in check, if it occurs too early during infection or if it is too great during later infection, PD-1 signaling may actually impair host defense and microbial clearance and worsen outcome. Studies have now suggested that the blockade of PD-1 ligand (PD-L1)/PD-1 signaling can restore lymphocyte host defense function and may be protective under some conditions. In one study, PD-1 knockout animals were resistant to cecal-ligation and puncture (CLP)-induced septic shock. In other studies, provision of an anti PD-1 or anti PD-L1 antibody both very early after the onset of infection (3 hours after induction of peritonitis) and much later (up to 5 days after induction of peritonitis) provided protection. One group has also shown that PD-1 blockade leads to a reduction in peritoneal lavage bacteria colony counts in conjunction with higher plasma levels of inflammatory cytokines. Thus, controlling PD-1 expression holds promise in the treatment of infection and sepsis. However, understanding under which conditions such inhibition will have beneficial as opposed to harmful effects is important for determining how widely PD-1 inhibition might be applied during clinical infection. Because the severity of underlying infection is one condition that varies substantially in patients presenting with sepsis and has an important impact on the efficacy of other immune-modulators, determining whether PD-1 inhibition has beneficial effects with both mild and severe infection is a critical question. Also, because, PD-1 inhibition has been studied exclusively in peritoneal infection models, determining whether it is also beneficial in the context of pulmonary infection, the most common cause of sepsis clinically, is important. To test these questions, in the present study we plan to compare the effects of an anti-PD-1 mAb versus placebo in mice challenged with intratracheal (IT) doses of S. aureus (SA) bacteria (i.e. a pneumonia model) designed to produce either low or high mortality rates (25% mortality, mild infection vs. 75%, severe infection respectively). The study will be done in 2 parts. The first part (Part 1) will include experiments to determine the doses of bacteria that will produce the low or high mortality rates that we seek and to also confirm that IT infection alters PD-1 expression on circulating and tissue immune cells. The second part (Part 2) will include two groups of experiments. One group will compare the effects of anti-PD-1 mAb versus control on survival in mice challenged with IT SA low or high doses and observed for up to 168h. The other group will compare the effects of these two treatments on blood and lung bacteria counts, cytokine levels, PD-1/PD-L1 tissue expression, tissue injury scores and other measures.